i#2080 sigreturn: set segment registers for thread_set_self_context
i#2080 sigreturn: set segment registers for thread_set_self_context

Sets the segment registers in the fake signal context used with sigreturn
to set a thread's context.  The registers are set to the current values.
i#2095 covers re-instating application values for application context
targets.

Review-URL: https://codereview.appspot.com/316970043

AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio